The fabrication of microelectronic, optoelectronic, and photonic devices typically involves numerous steps, including layer formation, dopant implantation, and selective pattern processing-- resulting in a desired structure to be electrically contacted and/or optically accessed. In this context, special attention will be paid in the following to pattern processing as applicable, e.g., in pattern etching, and especially to the patterning of a mask layer prior to processing of underlying material.
On a material to be processed, e.g., by etching, a mask or "resist" layer may be formed of a material which is sensitive to suitable radiation such as, e.g., visible or ultraviolet light, electrons, x-rays, or ions. After such a material has been selectively exposed to radiation, exposed or unexposed areas may be selectively removed by a chemical agent which leaves complementary areas essentially unaffected.
In the practice of such methods, the choice of mask material is preferably based on radiation response. However, there are instances in which material to be processed is coated with an auxiliary layer whose material composition is selected on the basis of different criteria: for example, in so-called epitaxial lift-off processing of an epitaxially grown layer, a support layer material may be chosen mainly on account of desirable properties including a low Young's modulus, a high coefficient of expansion, high adhesion, and high chemical durability.
Epitaxial lift-off processing involves growth of a desired film on a first, auxiliary or growth substrate, followed by removal of such film onto a second, desired device substrate. Such processing is receiving attention, e.g., as an alternative to lattice-mismatched epitaxial growth, the combination of silicon and Group III-V technologies currently being of particular interest. For example, as disclosed by E. Yablonovitch et al., "Extreme Selectivity in the Lift-off of Epitaxial GaAs Films", Applied Physics Letters 51 (1987), pp. 2222-2224, a gallium arsenide layer can be grown on an intermediary aluminum arsenide layer on a gallium arsenide substrate, and the grown gallium arsenide layer can be lifted off upon undercut etching--i.e., upon chemical dissolution of the intermediary layer.
Epitaxial lift-off is facilitated by use of a support layer which is formed on the layer to be lifted off, and it was found that a material known as Apiezon-W is particularly suited for this purpose. Indeed, use of such material results in preferred edge-curling during undercut-etching; see U.S. Pat. Nos. 4,846,931 and 4,883,561, issued to T. J. Gmitter et al. on Jul. 11, 1989 and Nov. 28, 1989, respectively. For exemplary uses of resulting films as bonded to desired substrates see, e.g., E. Yablonovitch et al., "Regrowth of GaAs Quantum Wells on GaAs Liftoff Films `Van der Waals Bonded` to Silicon Substrates", Electronics Letters, Vol. 25 (1989), p. 171; and E. Yablonovitch et al., "Double Heterostructure GaAs/AlGaAs Thin Film Diode Lasers on Glass Substrates", IEEE Photonics Technology Letters, Vol. 1 (1989), pp. 41-42.
Further with respect to epitaxial lift-off processing, as single-crystal layers typically are incorporated in devices or "chips" which are considerably smaller than a growth substrate or "wafer", and since lift-off of epitaxially grown layers is facilitated when the size of such layers does not exceed a practical limit, it is desirable to provide a means for partitioning a grown semiconductor layer prior to epitaxial lift-off. However, as an instance of a difficulty mentioned above, such partitioning is impeded in the presence of a support layer which is not amenable to patterning by selective irradiation. The invention described below is motivated by the desire for a patterning method which is applicable in this situation, and which does not rely on the use of radiation.